Launched April 01, reached Mars Oct. 01. Two-hour circular, polar mapping orbit established by Feb p.m. day/5 a.m. night

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1 Results from MARIE C. Zeitlin LBNL & NSBRI T. Cleghorn, F. Cucinotta, P. Saganti NASA JSC V. Andersen, K. Lee, L. Pinsky University of Houston W. Atwell Boeing Company, R. Turner ANSER

2 Odyssey Mission Launched April 01, reached Mars Oct. 01. Two-hour circular, polar mapping orbit established by Feb p.m. day/5 a.m. night THEMIS and GRS turned on. MARIE turned on March 2002, recovered after anomaly in cruise.

3 Mars Radiation Map GCR Calculated skin dose on surface, using HZETRN model. Unshielded doses ~ 1000 times greater on Mars than Earth s surface. Annual dose-equivalent (Sv/yr)

4 MARIE Silicon Detector Stack 1-mm Si detectors A1 & A2 define trigger, 30 FOV cone, proton E > 30 MeV to reach A2. No forward/backward distinction w/o Cerenkov. On trigger, read all detectors. 5-mm thick Si detectors B1-B4 provide high-resolution de/dx measurements. At these energies, de/dx in silicon tissue dose. Counts in A1, A2, B1, B2 recorded, stored per minute.

5 MARIE s Dirty Laundry Instrument has many problems PSD s noisy, unreliable Trigger threshold not well understood. Cerenkov not quite dead but close (might be fixable) Preamplifier gains too high, saturation at ~ 60 MeV deposited in A s & at ~ 320 MeV for B s. Can only handle about 3 events/second. Require t between events > 0.4 sec in analysis. Substantial deadtime corrections especially for SPEs. Only possible gain adjustment: very limited changes to bias voltages (lose-lose on signal:noise). Some improvements possible w/new software

6 Orientation MARIE bore-sight in s/c anti-velocity direction. FOV perpendicular to nadir vector, does not include Mars. Positioning & orientation non-critical for isotropic radiation. Considerable mass (> 100 g cm -2 ) behind.

7 Detection of GCR HZE See 1:2:1:2 ratio of B:C:N:O at high E (~1 GeV/amu) Upper limit for GCR ~ charge for high E. Higher Z or lower energy saturation. Stopping particles saturate if Z > 3. Result: dose & dose equivalent systematically underestimated. HZETRN predicts ~ 15% of dose due to Z > 12

8 Comparison with ACE/CRIS ACE/CRIS in near-earth orbit. ACE reports flux of protons with E > 30 MeV, good match. Recent quiet-time flux ~ 0.57 cm -2 sr -1 s -1 MARIE ~ 0.35 cm -2 sr -1 s -1 Need to characterize MARIE geometry, trigger threshold, etc., better Can t yet directly compare heavy-ion fluxes due to different E sensitivity. MARIE identifies high-e ions, ACE/CRIS low-e

9 Flux & Geometry Factor Issues Probable explanation for low flux result from MARIE: A1 & A2 have large edge regions of inefficient charge collection. Test of A efficiency: select heavy ions in B1+B2, look at A. Result 40% of these events have small E in A1 (poor correlation w/b signals). Similar for A2, 30% bad.

10 Selected Z > 4 in B2 vs. B1 scatter plot. ~40% of events in indicated region.

11 Flux & Geometry Factor, more Possibly, significant numbers of protons are missed due to trigger threshold. Recent study suggests we see protons up to 1 GeV or higher energy. Threshold was set by trial-and-error to get acceptable data rate. No ground data to tell us correlation between digital control value & true energy.

12 Q And Saturation Problem Amplifier saturation Q(L) LET [kev/micron] Everything above charge 12 (LET > 30 kev/µm) registers as charge 12 (LET = 30 kev/µm).

13 Measured Dose Rate vs. HZETRN Quiet-time dose agrees well w/model. Data normalization uncertain to ±20%.

14 GCR Dose Equivalent GCR dose ~ 210 µgy/day 16-month avg ± 0.14 Sv/yr. ~ 10% from SPEs. Used <Q> = 5 from HZETRN. Data <Q> = 3.5 due to saturation problem. Can bracket <Q> w/data*, find 3.5 < Q < 7. * All events with LET > 30 kev/µm assigned LET=100 kev/µm.

15 First Detailed SPE Data From Mars Several SPE s since March Earth-Sun-Mars angle has varied, was 100 to 180 for Observation of back-sided SPEs. Later in 03, Earth & Mars magnetically connected to same region on sun. Compare timing, intensity to GOES.

16 Earth-Mars Comparisons MARIE has seen: Events when nothing was seen at Earth. Events well-correlated w/near-earth observations. Times of no enhancement when an event was observed near Earth.

17 July 2002 Event MARIE & GOES-8 correlated despite nearly 180 Earth-Sun-Mars angle. Dose rate up ~ factor of 50. MARIE data acq swamped (counters o.k.) MARIE Event data Dose rate mrad/day MARIE A2 counter data GOES-8 protons E > 30 MeV (arbitrary normaliztion) Julian Day Day of Year (UT)

18 Two-hour Structure in SPE Intensity Periodic structure related to Odyssey s orbit regular peaks at North Pole + a few others. Not yet understood. Also seen by GRS & Mars Global Surveryor electron reflectometer. counts/minute MARIE A1 counter Odyssey Latitude Day of Year (2003)

19 MARIE & GRS ULD 6000 Count rate (arbitrary units) ULD = GRS gamma crystal upper level discriminator. Threshold energy unknown Day of Year 0

20 Oscillations vs. energy A2 B1 A1 A1 counter requires proton E > 20 MeV. A2 requires E > 30 MeV. Oscillations strongest in MeV protons, esp. later in event.

21 Relation to Magnetism? Martian magnetism is complicated. Field lines from surface extend to high altitudes, strength comparable to IMF. Mars Global Surveyor magnetometer superb data set thanks to extended aerobraking campaign.

22 H ~ 2.5 larger in Mars orbit than ISS. ISS orbit inside geomagnetosphere + shielding. July 02 SPE contributed only ~ 35% to H. Q=1 for protons. Contribution even less w/shielding.

23 GRS Physics Measure γ-rays & neutrons coming up. Made by nuclear interactions of incident protons. Combined γ + neutron data reveals elemental composition of top meter of soil.

24 GRS discovery of polar water ice H signal in γ-ray spectrum correlates w/lack of neutrons. Don t see H at north pole due to CO 2 cap. Seasonal, reverses in Northern Summer.

25 Neutron dose Neutrons on surface from below & above. GCR + soil nuclear interactions produce a neutron flux (upward-going). GCR + atmosphere nuclear int s produce neutrons (downward-going). GRS data w/model (MCNP) can be used to estimate dose of upward neutrons. NASA-LaRC model predicts ~ 0.02 Sv/yr. Detector on lander should have neutron sensitivity possibilities for 07, 09.

26 Mars Mission Risk Assessment Scenario: 1 yr. transit, 2 yrs. on Mars. Mitigating factors on surface: Charged particles from above only (4π in transit). Atmosphere provides some shielding, esp. from SPE. Worst exposure is in transit. Total ~ 1 to 1.5 Sv, dominated by GCR.

27 Summary MARIE is working well, returning first detailed radiation data from Mars. Improvements planned (event rate, C det., etc.). Good agreement between HZETRN and dose rate during quiet time. Data normalization still uncertain to ± 20%. Nominal mission ends 8/04, extension likely. Spacecraft in great shape, lots of fuel. Solar activity declining, minimum in 06-07, GCR increase expected. Mission could go into 08.